Magnetic random access memories (MRAMs) generally include magnetic tunnel junction (MTJ) elements, which serve as storage elements, in respective memory cells. Each MTJ element includes a reference layer of a magnetic material with a fixed magnetization direction, a storage layer of a magnetic material with a changeable magnetization direction, and a tunnel barrier layer (nonmagnetic layer) disposed between the reference layer and the storage layer. The “fixed” state of the magnetization direction of a magnetic material means that the magnetization direction of the magnetic material does not change after a write current is caused to flow between the reference layer and the storage layer. The “changeable” state of the magnetization direction of the magnetic material means that the magnetization direction of the magnetic material may change after a write current is caused to flow between the reference layer and the storage layer. The MTJ element is configured such that one of the reference layer and the storage layer, for example the reference layer, is disposed on a substrate, the tunnel barrier layer is disposed on the one of the reference layer and the storage layer, and the other of the reference layer and the storage layer, for example the storage layer, is disposed on the tunnel barrier layer.
A contact plug may be disposed on the storage layer, and a wiring line connecting to the contact plug is disposed on the contact plug in the MTJ element. The cross section of the contact plug perpendicular to the direction from the storage layer to the wiring line has substantially the same size at any point from the storage layer to the wiring line. As a result, a constant current flows in the plane of the MTJ element connecting to the contact plug.
During the manufacture of the MTJ element, the reference layer, the tunnel barrier layer, and the storage layer, which form a multilayer structure, are patterned in accordance with the planar shape of the MTJ element. During the patterning, each side portion of the multilayer structure is damaged for a distance of about 2 nm toward the inside of the multilayer structure. Electrical current flowing through the damaged portion does not effectively contribute to the spin transfer torque magnetization switching during a write operation of the MTJ element. If the size of the MTJ element is 20 nm or less, the ratio of the damaged portions to the entire cross-sectional area of the MTJ element becomes relatively high. This degrades write operation characteristics as compared to those of a larger MTJ element. In order to maintain the operation characteristics of an MTJ element with the size of 20 nm or less, the amount of current flowing through edge portions on the sides of the MTJ element needs to be decreased to reduce the ratio of the current flowing through the damaged portions.
An option to reduce the current density at the edge portions on the sides of the MTJ element is to restrict the current path for the current flowing into the MTJ element. The current path may be narrowed to the central portion of the MTJ element by reducing the cross-sectional area of the contact plug relative to the cross-sectional area of the MTJ element at the junction surface between the top surface of the MTJ element and the contact plug. As a result, the ratio of the current flowing through the central portion of the MTJ element increases. This allows the current flowing through the central portion of the MTJ element, which does not have damage, to effectively contribute to the spin transfer torque magnetization switching. As a result, a high-speed and low-current spin transfer torque magnetization switching operation is made possible. This improves the write characteristics of the MTJ element.
On the other hand, the contact plug serving as a current path may also serve as a heat conduction path. Therefore, simply narrowing the current path may lead to an increase in temperature of the MTJ element in operation since this prevents the Joule heat that is mainly generated at the tunnel insulating film of the MTJ element from being released easily. This causes a problem in that a read disturb error (erroneous writing of a cell during a read operation performed on another cell) is likely to occur during a read operation of the MTJ element. The increase in temperature of the MTJ element may also leads to a decrease in magnetization switching current and a decrease in resistance change rate (MR ratio) of the MTJ element. If the MR ratio decreases, the read current should be increased to secure the read signal intensity. As a result, the margin in the read operation decreases, and a read disturb may occur.
The narrowing of the current path may also cause a further problem of a decrease in the read signal intensity of the MTJ element since the contact resistance between the MTJ element and the contact plug increases.